The present invention relates to a method and an apparatus for forming "unevenness" on an outer surface of a synthetic resin-coated steel pipe.
The term "unevenness" used herein means unevenness formed of groove portions and ridge portions alternately arranged as shown in FIGS. 3 and 4 by numerals 20 and 21.
Synthetic resin-coated steel pipes, for example, are provided for marine bottom use by surrounding the outer surface of these pipes with concrete for the purpose of weight coating or protection. However, the synthetic resin-coated steel pipes are relatively hard and smooth on the outer surface, so that the bonding strength between the synthetic resin outer surface and the concrete surface is low. Accordingly, there is the possibility that the concrete surface and the synthetic resin outer surface may become separated owing to the external force acting in an axial direction of the steel pipe.